Cops2 Promotes Pluripotency Maintenance by Stabilizing Nanog Protein and Repressing Transcription
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www.nature.com/scientificreports OPEN Cops2 promotes pluripotency maintenance by Stabilizing Nanog Protein and Repressing Received: 30 March 2016 Accepted: 10 May 2016 Transcription Published: 26 May 2016 Weiyu Zhang1,*, Peiling Ni1,*, Chunlin Mou1,*, Yanqin Zhang1,*, Hongchao Guo1,2,*, Tong Zhao1, Yuin-Han Loh2,3 & Lingyi Chen1,4 The COP9 signalosome has been implicated in pluripotency maintenance of human embryonic stem cells. Yet, the mechanism for the COP9 signalosome to regulate pluripotency remains elusive. Through knocking down individual COP9 subunits, we demonstrate that Cops2, but not the whole COP9 signalosome, is essential for pluripotency maintenance in mouse embryonic stem cells. Down- regulation of Cops2 leads to reduced expression of pluripotency genes, slower proliferation rate, G2/M cell cycle arrest, and compromised embryoid differentiation of embryonic stem cells. Cops2 also facilitates somatic cell reprogramming. We further show that Cops2 binds to Nanog protein and prevent the degradation of Nanog by proteasome. Moreover, Cops2 functions as transcriptional corepressor to facilitate pluripotency maintenance. Altogether, our data reveal the essential role and novel mechanisms of Cops2 in pluripotency maintenance. Embryonic stem cells (ESCs) are able to self-renew indefinitely, and have the potential to differentiate into all types of cells in the adult organism. The unique property of ESCs, namely pluripotency, renders ESCs as a prom- ising cell source in regenerative medicine and cell replacement therapy. Transcriptional regulation plays a critical role in pluripotency maintenance of ESCs1,2. Previous studies found that three transcription factors Oct4, Nanog and Sox2 regulate each other and form a core transcriptional regulatory circuitry underlying pluripotency main- tenance3. This core regulatory circuitry not only activates the expression of pluripotency-associated genes, but also suppresses the expression of differentiation-related genes3. Meanwhile, many other pluripotency associated transcription factors and coactivators, including Klf4, Sall4, Esrrb and Ncoa3, regulate the three genes of the core regulatory circuitry, forming an expanded pluripotency regulatory network and allowing signaling pathways integrated into transcriptional regulation4–8. In addition, the core components of the pluripotency network are regulated at protein level through post-translational modifications. For example, phosphorylation of Ser/Thr-Pro motifs of Nanog promotes the interaction between Nanog and Pin1, and stabilizes Nanog protein9. Both Oct4 and Sox2 are modified with O-linked-N-acetylglucosamine (O-GlcNAc). O-GlcNAcylation of Thr 228 enhances the transcriptional activity of Oct4, and regulates the functions of Oct4 in maintaining ESC self-renewal and reprogramming somatic cells10. The COP9 signalosome (CSN), composed of 8 subunits (Cops1 to Cops8), is highly conserved from yeast to human11–13. The most studied CSN function is to regulate protein degradation. The CSN suppresses the activity of the cullin-RING-E3 ligases (CRL) through deneddylation of cullins, thus enhancing protein stability14,15. It also regulates the ubiquitin ligase COP1, consequently the degradation of COP1 substrates16. Moreover, CSN-associated deubiquitinating enzymes, Ubp12 in yeast and USP15 in mammals, may stabilize the adaptor subunits of CRL and 1State Key Laboratory of Medicinal Chemical Biology, Collaborative Innovation Center for Biotherapy, 2011 Collaborative Innovation Center of Tianjin for Medical Epigenetics, Tianjin Key Laboratory of Protein Sciences and College of Life Sciences, Nankai University, Tianjin 300071, China. 2Epigenetics and Cell Fates Laboratory, A* STAR Institute of Molecular and Cell Biology, 61 Biopolis Drive Proteos, Singapore 138673, Singapore. 3Department of Biological Sciences, National University of Singapore, Singapore. 4State Key Laboratory of Molecular Oncology, Cancer Institute/Hospital, Chinese Academy of Medical Sciences, Beijing 100021, China. *These authors contributed equally to this work. Correspondence and requests for materials should be addressed to Y.-H.L. (email: yhloh@ imcb.a-star.edu.sg) or L.C. (email: [email protected]) SCIENTIFIC REPORTS | 6:26804 | DOI: 10.1038/srep26804 1 www.nature.com/scientificreports/ Iκ Bα , respectively, through deubiquitination17,18. In addition to regulation of protein degradation, the CSN is also involved in transcriptional regulation, protein phosphorylation and subcellular distribution19–23. While the CSN functions as a complex, CSN subunits may also have their own functions independent of the CSN complex. For example, Alien, a variant of Cops2, has been demonstrated to be a transcriptional corepressor24,25. Through a whole-genome RNAi screening experiment, it has been shown that down-regulation of CSN subu- nits, COPS1, COPS2 and COPS4, reduces the expression of the OCT4-GFP reporter in human ESCs, indicating a role of the CSN in pluripotency maintenance26. Consistent with this observation, some CSN subunits are required for mouse embryo development. Homozygous knockout of Cops2, Cops3, Cops5, Cops6, or Cops8 in mice, leads to early embryo death27–31. No ESCs could be derived from Cops2 or Cops8 null blastocysts, implying that the CSN is involved in pluripotency establishment29,30. Yet, Cops2, Cops3, Cops5, Cops6, or Cops8 knockout embryos die at different embryonic days ranging from day 6.5 to 8.5, implying that individual CSN subunits have their own biological functions, in addition to the function of the CSN. It is not clear whether the whole CSN complex or individual CSN subunits are required for pluripotency maintenance, and how the CSN or individual CSN subu- nits contribute to pluripotency maintenance. To elucidate the function and mechanisms of the CSN in pluripotency maintenance, we knocked down individual CSN subunits in mouse ESCs, and found that only Cops2 is essential for pluripotency maintenance in mouse ESCs. We further demonstrated that Cops2 stabilizes Nanog protein through direct interaction. In addition, Cops2 func- tions as a transcriptional corepressor to suppress gene expression, including 2-cell-stage embryo specific (2C) genes. In summary, our data revealed that Cops2, but not the CSN, is required for pluripotency maintenance in mouse ESCs. Results To clarify the role of CSN subunits in pluripotency maintenance, we examined the expression of pluripotency genes, Nanog, Oct4, and Sox2, upon knocking down each CSN subunit by shRNA in mouse ESCs (Fig. 1a). In con- trast to the RNAi screening data in human ESCs26, only knockdown (KD) of Cosp2, but not any other CSN sub- units, reduces the expression of Nanog and Oct4 mRNA (Fig. 1b). To rule out the possibility of shRNA off-target effect, the regulatory effect of Cops2 on Nanog and Oct4 at both RNA and protein levels was further validated with another shRNA targeting Cops2 (Fig. 1c,d). In addition, differentiation markers of three germ layers and the trophectoderm (TE), except for the ectodermal marker Pax6, are up-regulated upon Cops2 KD in ESCs (Fig. 1e), implicating compromised pluripotent status in Cops2 KD ESCs. To further characterize the role of Cops2 in ESCs, we attempted to establish stable Cops2 KD ESCs. ESCs were transfected with plasmids expressing shRNAs targeting GFP and CSN subunits. Much less Cops2 KD colonies (less than 10 colonies per experiment) grew up after 7–10 day puromycin selection, compared to other CSN subunits and GFP KD ESCs (ranging from 50 to 280 colonies) (Supplementary Table S1). Moreover, Cops2 is inefficiently knocked down in the survivingCops2 KD ESC clones (Supplementary Fig. S1). These data imply that Cops2 is essential for the self-renewal of ESCs. We then switched to knock down Cops2 transiently in ESCs. KD of Cops2 impairs the alkaline phosphatase (AP) positivity in ESCs, while Cops8 KD does not affect the expression of AP (Fig. 2a). In addition, Cops2 KD slows down the proliferation of ESCs, likely due to G2/M arrest, but not cell apoptosis (Fig. 2b–d). Consistent with the slower proliferation rate of Cops2 KD ESCs, the colony forming ability of ESCs is also reduced upon Cops2 KD (Fig. 2e). All these data indicate that KD of Cops2 compromises the self-renewal of ESCs. Next, we sought to characterize whether Cops2 KD affects the differentiation potential of ESCs. Embryoid bodies (EBs) formed by Cops2 KD ESCs are smaller than control GFP KD EBs (Fig. 2f). Moreover, most of the differentiation markers are not fully activated in Cops2 KD EBs, particularly the ectodermal markers Nestin and Pax6 (Fig. 2g), implying that the differentiation potential of ESCs is impaired upon Cops2 KD. To address whether Cops2 plays a role in somatic cell reprogramming, Cops2 was knocked down or over- expressed in mouse embryonic fibroblasts (MEFs), which were co-infected with retroviruses expressing Oct4, Sox2, Klf4 and c-Myc. Cops2 KD reduces the reprogramming efficiency, while overexpression ofCops2 increases the number of AP positive colonies after reprogramming (Fig. 2h,i), implicating that Cops2 facilitates Yamanaka factors to reprogram somatic cells. It is noticeable that the down-regulation of Nanog is more eminent at protein level than at RNA level (Fig. 1c,d), implying that Cops2 might regulate Nanog protein stability. To understand how Cops2 regulates the expression of Nanog, through transcriptional regulation or by affecting Nanog protein stability, we first performed luciferase reporter assay using a 6-kb Nanog promoter. KD of Cops2 does not affect